Summary:
In the anaerobic respiratory chain formed by NADH dehydrogenase and nitrate reductase the transfer of electrons from NADH to nitrate is coupled to the generation of a proton-motive force (H+/e- = 3) across the cytoplasmic membrane ([Cole68, Garland75, Haddock75] and see [Unden97]).

E. coli K-12 contains two NADH dehydrogenases - energy conserving NDH-I (shown in this pathway) and NDH-II which does not contribute to the proton gradient; both enzymes appear to be involved in anaerobic nitrate respiration [Bongaerts95, Tran97]. By analogy to the related enzyme from mitochondria, NDH-I is thought to function as a proton pump translocating 4H+ per NADH oxidised (2e-) [H+/e- = 2] however a lower ratio of 3H+/2e- has also been proposed [Bogachev96, Wikstrom12]. Nitrate induces the expression of the nuo operon (encoding NDH-I) in a NarL dependent manner [Bongaerts95]

E. coli K-12 also contains two energy conserving (H+/e- = 1) nitrate reductases. Expression of nitrate reductase A (NRA) occurs in response to high levels of nitrate in the environment whereas expression of nitrate reductase Z (NRZ) is not dependent on nitrate levels or anaerobiosis ([Wang99b] and see [Bonnefoy94]).

Quinones are the obligate redox carriers during anaerobic nitrate respiration; the concentration of menaquinone increases in cells grown anaerobically with nitrate while the concentration of ubiquinone decreases (as compared with cells grown aerobically) [Wallace77]. Nitrate reductase A can use both menaquinol (shown in this pathway) and ubiquinol as electron donors [Wissenbach90, Wissenbach92]. In anaerobic growth with nitrate the major quinone is demethylmenaquinone (DMK) [Unden88]; an E. coli strain containing only demethylmenaquinone is unable to grow with nitrate as terminal reductase [Wissenbach92].